SESOIL is a seasonal compartment model which simulates long-term pollutant fate and migration in the unsaturated soil zone. SESOIL describes the following components of a user-specified soil column which extends from the ground surface to the ground-water table.

Hydrologic cycle of the unsaturated soil zone.

Pollutant concentrations and masses in water, soil, and air phases.

Pollutant migration to ground water.

Pollutant volatilization at the ground surface.

Pollutant transport in washload due to surface runoff and erosion at the ground surface.

SESOIL estimates all the above components on a monthly basis for up to 999 years of simulation time. SESOIL can be used to estimate the average concentrations in ground water. The soil column may be composed of up to four layers, each layer having different soil properties which affect the pollutant fate. In addition, each soil layer may be subdivided into a maximum of 10 sublayers in order to provide enhanced resolution of pollutant fate and migration in the soil column. The following pollutant fate processes are accounted for: Volatilization, Adsorption, Cation Exchange, Biodegradation, Hydrolysis and Complexation.

SESOIL can be used as a screening tool in performing exposure assessments. It was enhanced to allow one to specify the initial adsorbed soil concentrations at any sublayer. EPA uses the model to predict the behavior of pollutants in soil compartments for analyzing and prioritizing chemical exposures. A number of studies have been conducted on the SESOIL model including sensitivity analysis, comparison with other models, and comparisons with field data. SESOIL has been applied in risk assessments concerning direct soil liquefaction, incineration of hazardous waste, the transport of benzene to groundwater, to soil cleanup levels in California, and to site sensitivity ranking for Wisconsin soils for the Wisconsin DNR.

SESOIL accepts time-varying pollutant loading. For example, it is able to simulate chemical releases to soil from a variety of sources such as landfill disposal, accidental leaks, agricultural applications, leaking underground storage tanks, or deposition from the atmosphere. Other potential applications include long-term leaching studies from waste disposal sites, pesticide and sediment transport on watersheds, studies of hydrologic cycles and water balances of soil compartments. SESOIL can also be used to estimate the effects of various site management/design strategies on pollutant distributions and concentrations in the environment.

SESOIL INPUT DATA

The input data required by SESOIL consists of five types: climate, soil, chemical, application and washload. All the parameters of the five input files are listed in the Table below.

Climate Data - consists of monthly rainfall and climatic statistics for a typical year for the rain gauge of interest.

Soil Data - consists of soil parameters averaged over the entire soil zone such as the bulk density of the soil, intrinsic permeability, etc.

Chemical Data Ð requires the solubility and the adsorption coefficient of the chemical.

Washload Data Ð optional and contains data used by SESOIL to calculate washload transport, the migration of the pollutant adsorbed to eroding soil particles.

SESOIL DATABASE SUPPORT

The SESOILClimate Database consists of 3,225 weather stations throughout the continental U.S. and is available for purchase by State or the entire U.S. The data originally came from the National Climatic Data Center (NCDC) of the National Oceanographic and Atmospheric Administration (NOAA). It consists of all the monthly statistical parameters required by SESOIL (rainfall information from hourly precipitation stations and evaporation-related information from monthly State climatic division means) accessed by latitude/longitude coordinates of the U.S. An additional variable, ALBEDO, which represents the monthly short-wave albedo of the land surface is also incorporated into the data. This database can be accessed by providing the location coordinates of the site. The interface brings up a list of the 50 closest climatic stations to the site location that the user enters.

The SESOIL Zip Code Database contains the zip codes and the corresponding latitude/longitude coordinates for all of the U.S.

The SESOILSoils-5 Database provides soil information required by SESOIL for over 14,000 soil series across the U.S. and comes from the USDA Soil Conservation Service's (SCS) Soils-5 database. Single variables, i.e., bulk density of the soil, effective porosity, erodibility factor, and disconnectedness index; and layer-dependent variables, i.e., thickness, intrinsic permeability, Organic Carbon (OC) content, and pH are retrieved from the database. The Soils-5 Database can be accessed through the Soils database button by specifying the first two letters of the soil name.

The SESOIL interface provides measured properties for eight of the most commonly-modeled chemicals. These files are organized by the Chemical Abstracts Service (CAS) number of the chemical along with the .MSR file extension for the file. The CAS numbers and the chemical names of the eight chemicals are provided below:

CAS Numbers

Chemical Name

71432

Benzene

71556

1,1,1-Trichloroethylene

79016

Trichloroethylene

91203

Naphthalene

100414

Ethylbenezene

108883

Toluene

118741

Hexachlorobenzene

127184

Tetrachloroethylene

SESOIL OUTPUT

SESOIL creates an output file which contains monthly results for hydrologic cycle components, pollutant mass distribution, and pollutant concentration distribution for each layer or sublayer. SESOIL also generates a SESOIL-AT123D link file which contains pollutant mass-to-ground-water leachate quantities entering the aquifer from the unsaturated zone. This file can be used as input to the AT123D ground-water model in RISKPRO. Within RISKPRO, line graphs of "concentration vs time" at any specified depth within the unsaturated zone may be created from SESOIL results. " Pollutant-front-depth vs. time" may also be created.